1. Field of the Invention
The present invention relates to voltage controlled oscillators and more particularly to voltage controlled oscillators intended for use in phase-locked loop systems. Still more particularly, the present invention is directed to a voltage controlled oscillator for use in a phase-locked loop data separation circuit for use in disc drive systems.
Typically, information is recorded on magnetic discs in disc drive systems in the form of data pulses combined with clock pulses. In order to read information from the disc, signals from a magnetic head are applied to a data synchronization circuit which generates a recovered clock signal based upon the detection of data and clock pulses. The recovered clock signal corresponds to the pulse rate of the signals read from the disc and is used to separate the data and clock pulses. The generation of the recovered clock signal is accomplished by means of a phase-locked loop system. The phase-locked loop system typically employs a voltage controlled oscillator which is controlled so as to generate an output signal having a frequency equal to the data rate. The input to the voltage controlled oscillator is a control signal representative of the phase error between the incoming signal from the disc and the output of the phase-locked loop.
2. Description of the Prior Art
Prior art voltage controlled oscillators for use in data separators have typically been comprised of an inverting amplifier connected to a resonant feedback network such as an LC network. A varactor forms part of the feedback network and the voltage applied to the varactor determines the oscillation frequency. The output of the amplifier is the oscillator output. Although generally acceptable, such oscillators have several drawbacks. First, the duty cycle of the oscillator is not balanced, i.e., the high and low signal levels for each cycle are not of equal duration. Secondly, such oscillators typically have a start-up delay before they reach the desired operating phase due to small, slowly increasing amplitude of the oscillations.